Hi Geoff and Hortense Ingram,
Thank you for reaching Microsoft Community.
I would like to confirm first regarding with the model of your Surface. Is it a Surface Go, Surface Go 2 or Surface Go 3? To confirm, you may enter the serial number here: Check Warranty Page - ROBO SelfServe Portal (surface.com).
Regarding with the concern, how about the external speaker or using a wired headphones, do you still do not get any sound from the device? Please note that Airplane mode/Flight mode is a way for us to quickly turn off all wireless communications on the device including Bluetooth and does your Bluetooth headphones will stop working when Airplane mode is turned on. See: Turn airplane mode on or off (microsoft.com).
Kind regards,
Marrion